We describe some recent progress in understanding and formulating string theory which is based on extensive studies of strings in lower (D=2) dimension At the center is a large $W_{\infty}$ symmetry that appears most simply in the matrix model picture In turn the symmetry defines the dynamics giving Ward identities and the complete S-matrix The integrability aspect where nonlinear string phenomena emerges from linear matrix model dynamics is emphasized Extensions involving couplings of discrete topological fields to the tachyon are also described

The boundary state associated with the rolling tachyon solution on an unstable D-brane contains a part that decays exponentially in the asymptotic past and the asymptotic future, but it also contains other parts which either remain constant or grow exponentially in the past or future. We argue that the time dependence of the latter parts is completely determined by the requirement of BRST invariance of the boundary state, and hence they contain information about certain conserved charges in the system. We also examine this in the context of the unstable D0-brane in two dimensional string theory where these conserved charges produce closed string background associated with the discrete states, and show that these charges are in one to one correspondence with the symmetry generators in the matrix model description of this theory.

Fields and Symmetries of 2D Strings

Rolling Tachyon Boundary State, Conserved Charges and Two Dimensional String Theory

Two dimensional string theory is known to have an infinite dimensional symmetry, both in the continuum formalism as well as in the matrix model formalism. We develop a systematic procedure for computing the conserved charges associated with these symmetries for any configuration of D-branes in the continuum description. We express these conserved charges in terms of the boundary state associated with the D-brane, and also in terms of the asymptotic field configurations produced by this D-brane. Comparison of the conserved charges computed in the continuum description with those computed in the matrix model description facilitates identification of the states between these two formalisms. Using this we put constraints on the continuum description of the hole states in the matrix model, and matrix model description of the black holes solutions of the continuum theory. We also discuss possible generalization of the construction of the conserved charges to the case of D-branes in critical string theory.

Symmetries, Conserved Charges and (Black) Holes in Two Dimensional String Theory

The field content of the two dimensional string theory consists of the dynamical tachyon field and some nonpropagating fields which consist in the topological sector of this theory. We propose in this paper to study this topological sector as a spacetime gauge theory with a simple centrally extended $w_\infty$ algebra. This $w_\infty$ algebra appears in both the world sheet BRST analysis and the matrix model approach. Since the two dimensional centrally extended Poincar\'e algebra is naturally embedded in the centrally extended $w_\infty$ algebra, the low energy action for the metric and dilaton appears naturally when the model is truncated at this level. We give a plausible explanation of emergence of discrete states in this formulation. This theory is again the effective theory at zero slope limit. To include higher order $\alpha'$ corrections, we speculate that the whole theory is a gauge theory of a deformed $w_\infty$ algebra, and the deformation parameter is just $\alpha'$.

Fields and Symmetries of 2D Strings

Citations

Rolling Tachyon Boundary State, Conserved Charges and Two Dimensional String Theory

Symmetries, Conserved Charges and (Black) Holes in Two Dimensional String Theory

A proposal on the topological sector of 2d string

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